Primarily, uranium is obtained from uranium ores. A secondary source for uranium is sedimentary phosphate deposits. Most of these deposits, worldwide are uraniferous containing from about 50 to 200 ppm uranium. Both these source materials, uranium ore or phosphate rock, are often leached with sulfuric acid. While uranium ore gives soluble sulfates of uranium in a dilute (˜0.01 M) acidic leach solution, the phosphate rock produces more concentrated (>4 M) phosphoric acid which is used to make fertilizers and/or detergent phosphates.
During the wet process of producing phosphoric acid, many impurities in the phosphate rock including the uranium values and significant amounts of the rare earths in the rock get solubilized as complex phosphates. While the uranium values in the phosphoric acid can be recovered by solvent extraction from the phosphoric acid obtained at various stages of its preparation, rare earths remain in the phosphoric acid.
Generally, two types of phosphoric acids one called weak acid when P2O5 content is up to 30% by wt. (˜5.5 M) and another called strong acid when P2O5 content is between 30–55% by wt. (5.5–13 M) are available for the recovery of the uranium.
The efficiency and techno-economic feasibility of the recovery of uranium depends on the proper choice of the solvents employed for the type of phosphoric acid chosen. Further, the nature and the amount of the impurities such as fluorine, iron, aluminum, silica, magnesium, rare earths, and compounds of other minor elements solubilized in the phosphoric acid influence the purification/extraction steps to obtain uranium of desired quality.
U.S. Pat. No. 4,238,457(1980), U.S. Pat. No. 4,302,427(1981), and U.S. Pat. No. 4,778,663(1988) disclose the extraction involving D2EHPA-TOPO as extractant. This process is characterized by high selectivity. However, it uses a relatively expensive extractant component TOPO, which is also not readily available worldwide. This process uses a dual cycle process for extraction of oxidized uranium; but in view of the high synergism of TOPO and high selectivity, the co-extraction of iron and rare earths is not a particular feature of this process.
The known dialkyl phosphoric acid extraction or ‘DAPEX Process’ extracts uranium from acid leach solutions derived from uranium ores and not from phosphate rocks. This process has been described by Blake et. al. In ORNL—Report 2172, Jan 1957 for recovery of uranium by solvent extraction of leach solution obtained from sulfuric acid leached uranium ore and not from phosphate rocks.
The process uses dilute ˜0.1 M solution of D2EHPA and tri-n-butyl phosphate (TBP) in nearly equi-molar ratio diluted with kerosene for extraction of uranium. The absence of phosphate ions helps in efficient extraction of uranium with this extractant system. However, such extraction solvent is known to be not suitable for extraction of uranium in the process of phosphate ions. Thus, the DAPEX process could not be used for recovering pure uranium from weak or strong phosphoric acid available in phosphatic fertilizer industry.
Florin T. Bunus in his paper entitled “Determination of low levels of uranium in solutions obtained by acid attack on phosphate rock” by in Talanta, 1977,24, 117–120 suggests to check the application of the spectrophotometric and X ray fluorescence methods on the industrial phosphoric acid solutions, the extraction of uranium using 1.2 M D2EHPA with 0.15 M TBP in kerosene, from sodium chlorate oxidized (about 4.3 M) phosphoric acid. The uranium was stripped with 8.6 M phosphoric acid containing Fe(II). The Bunus procedure is basically an analytical method and does not apply to extraction from industrial phosphoric acid which include rare earths which contaminate the product and affect re-use of extracting solvent.
F. Bunus, I. Miu and R. Dumitrescu, in “Simultaneous recovery and separation of uranium and rare earths elements from phosphoric acid in a one-cycle extraction-stripping process”; Hydrometallurgy, 35(1994) 375–389) also known as the Romanian process proposes one cycle D2EHPA/TBP-HF process to produce a crude concentrate of uranium. The crude uranium cake is further refined in another plant. The process is, however, disadvantageous for the following reasons:                i. Use of corrosive and hazardous hydrofluoric acid (HF).        ii. Poor quality of both the products obtained namely the uranium and rare earths which therefore require further processing.        iii. The phosphoric acid contaminated with hydrofluoric acid is returned to the fertilizer plant. This requires special pollution control measures during fertilizer production.        
A two stage precipitation of uranium (the French process) is disclosed in “A New Unit for Purification of Uranium Solution in the Lodeve Mill”; G. Lyaudet, P. Michel, J. Moret and J. M. Winter, IAEA1987). This process particularly describes a multiple step process for recovering pure uranium peroxide from aqueous solutions obtained in the uranium leaching from concentrated uranium materials that are contaminated with organics, zirconium, molybdenum etc.
However, such a process is not relevant when the starting carbonate solution for recovery of uranium is obtained not directly from the leaching of uranium ores but obtained through a solvent extraction step from phosphoric acid where the separation of molybdenum and zirconium are not relevant.